Method for the preparation of doped oxide material

ABSTRACT

A method for preparing doped oxide material, in which method substantially all the reactants forming the oxide material are brought to a vaporous reduced form in the gas phase and after this to react with each other in order to form oxide particles. The reactants in vaporous and reduced form are mixed together to a gas flow of reactants, which gas flow is further condensated fast in such a manner that substantially all the component parts of the reactants reach a supersaturated state substantially simultaneously by forming oxide particles in such a manner that there is no time to reach chemical phase balances.

FIELD OF THE INVENTION

The invention relates to a method for the preparation of a doped oxidematerial.

An important usage of doped glass materials is light amplificationwaveguides, for example active optical fibres, whose light amplifyingproperties are based on utilizing stimulated emission. In order to makestimulated emission possible, the glass material in the core of theactive optical fiber, and possibly the cladding layer surrounding thecore, are doped with dopants, which are rare earth metals, for exampleerbium. In addition to optical fibers, the doped glass materials canalso be used in different kinds of optical planar waveguides.

BACKGROUND OF INVENTION

The active optical fibers are prepared by drawing glass into opticalfiber from a fiber preform, which fiber preform can be created inseveral different ways. A generally used manner for preparing a fiberpreform is to grow glass material around a mandrel, or a correspondingsubstrate arranged to rotate, by flame hydrolysis deposition, FHD. Whenthe above-mentioned growing is performed from the outer periphery of thefiber preform, it is often in this connection referred to as a so-calledOVD method (outer vapour deposition). The FHD method is also applied informing glass layers required in optical planar waveguides on a planarsubstrate.

In the FHD method, a hydrogen-oxygen flame is typically used as athermal reactor, and the glass forming base materials used in thepreparation of glass material, for example silicon or germaniumtetrachloride, are directed to the burner and the flame typically in avapour form. The dopants of glass material, such as, for example,erbium, are directed to the burner and the flame typically with carriergas as vapour or aerosol droplets, which are formed of the liquidcontaining dopants correspondingly either by vaporizing or by spraying.

Alternatively, according to the solution developed by the applicant, thedopants can be directed all the way to the burner in liquid form and beatomised as aerosol droplets, for example by using hydrogen flow, notuntil in the immediate vicinity of the flame. This method, which isdescribed more in detail, for example, in the applicant's earlierpublication WO 00/20346 and which can be considered a furtherdevelopment of the conventional FHD method, is later referred to asliquid flame spraying.

In the flame functioning as a thermal reactor in the FHD or liquid flamespraying method, the base materials and dopants further form aerosolparticles, which aerosol particles are guided onto the substrate to becoated, thus forming a doped porous glass material coating. Theseaerosol particles are often in literature referred to as “glass soot” inEnglish. When a suitable coating layer of porous glass material has beengrown on the mandrel or other substrate, the above-mentioned coatinglayer is sintered into a dense glass by heat-treating the substrate atan appropriate high temperature.

A so-called solution doping method is also known, in which method afiber preform grown of only base materials is dipped into a solutioncontaining dopants only after growing the fiber preform, beforesintering.

Rare earth metals dissolve poorly into quartz glass and require that,for example, the structure of SiO₂-based glass is changed by adding anappropriate oxide to the glass. Oxides suitable for the purpose are, forexample, Al₂O₃, La₂O₃, Yb₂O₃, GeO₂ or P₂O₅. Preferably this oxide isaluminium oxide Al₂O₃, which at the same time increases the refractiveindex of the glass.

When doping the core of optical fiber (or other waveguide) with a rareearth metal, an increase in the refractive index of the core in relationto the cladding layer is achieved at the same time by means of thealuminium oxide, which is necessary in order for the operating principleof the optical fiber to materialize. In the liquid flame spraying methodof the applicant, the aluminium is added by atomising aluminium chloridedissolved in a suitable liquid to the flame. Liquids suitable for thepurpose are, for example, water, organic solvents, such as ethanol,methanol, acetone, or mixtures of the above. Correspondingly, nitrate orchloride based sources dissolved in a liquid are used for rare earthmetals, such as erbium.

In the growing that takes place by means of the methods described above,when silicate/alumina glass are doped with rare earth metals, oneproblem is the inhomogeneous distribution of dopants into aerosolparticles forming glass coating. This is caused by e.g. the tendency ofdopants to form pairs. In a chemical balance, erbium does not dissolvein said materials as individual ions separate from each other. In a gasphase erbium aims to oxidize into form Er₂O₃ and in a solid phase erbiumaims typically to a phase system Al₅Er₃O₁₂+Al₂O₃ with aluminium. Inother words, with aluminium erbium aims to occur clustered in its ownphases. Even though the situation in a glass-like silica/alumina systemis more complex than described above, the above discussion offers a goodimpression on how erbium acts.

Especially when using the liquid flame method, most of aluminium and amajority of erbium aims to remain in the solid residual particle, whichis created from a liquid aerosol droplet when it “dries” in the flame,and wherein the abovementioned oxidation of materials into glass formingoxides takes place. Because of this, the fiber preform forming in theprocess typically includes at least two types of glass soot particles.Firstly, small Si-containing (or Ge-containing) particles, which areformed via condensation from vaporous base materials and theevaporation/drying following it. Secondly, aluminium and erbiumcontaining residual particles, which are typically larger than theseSi-particles. Because of these different types of particles, there is acrystallizing tendency in the glass material when it is sintered.

During sintering, a part of the crystals may also melt, which improvesthe homogeneity of the glass material. However, there is the risk thatremaining dopants, especially in the larger residual particles, do noteven then dissolve completely in the glass, in which case, when examinedon the small scale, the consequence is that the dopants are locallyinhomogeneously parted in the glass material. This weakens the lightamplifying properties of the glass.

On the other hand, in the case of a, for example, silicon wafer basedplanar waveguide, the temperatures used in sintering are more limitedthan in the case of a fiber preform meant for optical fiber. Thus,unwanted crystals causing scattering unavoidably remain in the readyglass coating even after sintering and because of the inhomogeneouscomposition of the glass material also the light amplifying propertiesof glass are unideal.

In all such processes, wherein glass soot particles and especiallyparticles containing dopants are not created substantially directly bycondensation via a gas phase, but larger liquid aerosol droplets are anintermediate phase, a problem is that different impurities also remain(encapsulate) in the residual particles forming from aerosol droplets.

SUMMARY OF THE INVENTION

It is the main aim of the present invention to present a completely newmethod for producing doped oxide material, with which method theabove-described problems occurring in the processes according to priorart are avoided.

With the method according to the invention, it is possible to producedifferent oxide materials. For example, it is possible to producemulticomponent oxide materials, wherein there are several reactants,whose respective parts are substantially equal respectively, such as,for example, bariumtitanate (BaTiO₃).

The method can also be used in producing such multicomponent oxidematerials, wherein there are several reactants, whose respectiveportions are of substantially different sizes respectively, such as, forexample, piezoelectric PZT (Pb(Zr_(1−x)Ti_(x))O₃), which has a highdielectric constant. (In the formula the parameter x determines theratio of zircon and titanium, and a typical value is, for example,0.45).

With the method according to the invention, it is also possible toproduce so-called doped oxide materials, wherein there are largerportions of base materials and smaller portions of dopants, such as, forexample, titanium oxide (TiO₂) doped with molybdenum (Mo), as well assome doped glass materials.

The aim of the invention is thus to make it possible to prepare dopedoxide material, which is more equal in quality than before, in whichoxide material the composition is on the micro level more homogeneousthan before and wherein the crystal structure is as desired. Thedifferent kind of properties of oxide material become more optimal thanbefore with the invention, in which case it is possible to manufacturebetter products than before from the oxide material.

The aim of an embodiment of the invention is to make it possible toprepare doped glass material, which is more equal in quality thanbefore, in which glass material there is no harmful crystallization andthe composition of the glass is also on the micro level more homogeneousthan before. In glass material formed in this manner, there is thus lessunwanted light scattering, which scattering causes attenuation/loss oflight in the light guides prepared from the glass material in question.The light reinforcing properties of the glass material also become moreoptimal than before with the invention, in which case it is possible tomanufacture better active light guides than before from the glassmaterial, for example active optical fibers.

A substantial basic idea of the invention can be considered to be thatall the reactants required in the preparation of doped oxidematerial,both the base materials and dopants are first brought to avaporous form, i.e. a gas phase. Condensation of reduced components fromthe gas phase to a liquid phase is performed extremely fast in such amanner that all components contained in the reactants and required informing doped material are brought substantially simultaneously to asupersaturated state, in which case the composition of liquid dropletsforming in this manner and solid particles forming immediately from themis made very homogeneous. The homogeneous composition of particlesrefers here to that, first of all, different particles have the samecomposition respectively, but also that the local inner composition ofan individual particle is homogeneous, i.e. in an individual particleall the components are equally divided over the entire volume of theparticle.

According to the invention, the above-mentioned fast condensation of thecomponents of the reactants is achieved either by fast oxidation ofreactants and/or by fast adiabatic expansion of the gas flow of thereactants.

The conditions according to the invention are arranged so that theparticles also solidify immediately after condensation, in which casethere is no time to reach chemical phase equilibrium.

By means of the invention, it is possible to prepare a doped oxidematerial, such as, for example, doped glass material, to be morehomogeneous in its composition than before, in which case, for example,in glass materials amplifying light and doped with rare earth metals,the amplifying properties can be optimised better than in prior art.When using, for example, erbium as a dopant, it is possible by means ofthe invention to prevent erbium from clustering, and erbium can bebrought to distribute more smoothly over the glass material, preferablyas individual ions. In the case of silico-based planar waveguides, theproblems resulting from crystallization of glass material and theunwanted scattering properties resulting from crystallization areavoided. Further, by means of the invention it is possible to avoid suchimpurities, which in the processes according to prior art tend tocapsulate in the inner parts of the residual particles.

The invention and some of its advantageous embodiments are describedmore in detail in the following, where it will also become more clear toa man skilled in the art what the advantages reached with the inventionare. The doped oxide material produced in the embodiment according tothe example presented by means of the figures is a doped glass material,and on the basis of the description connected to it, a man skilled inthe art can apply the invention also in producing other doped oxidematerials by possibly implementing minor changes in the embodimentaccording to the example.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows, in principle, a structure of a reactor according to theinvention in a perspective view,

FIG. 2 shows, in a principle view, a cross section of the reactoraccording to the invention, and

FIG. 3 shows, in a principled side-view, another reactor according tothe invention.

All the reactants required in the preparation of doped glass materialaccording to the invention, both the base materials (for example Si orGe) and dopants (for example Al and rare earth metals) are in thebeginning brought to a vaporous form i.e. the gas phase, byappropriately increasing the temperature of said materials and bychoosing an appropriate chemical composition for each reactant. Heatingthe reactants can be implemented with any manner apparent as such to aman skilled in the art. For example, silicon tetrachloride SiCl₄ can beused as the base material of glass material and aluminium and erbium asthe dopant, the latter either as nitrates or chlorides. The compoundsused as the sources of aluminium and erbium can be, for example,dissolved in appropriate liquids and evaporated further to a gas phaseby heating the solutions in question. In conveying the reactants broughtto the gas phase, it is possible to utilize appropriate carrier gases.

The base materials and dopants in a gaseous or reduced form are nextguided mixed together or still as separate gas flows B, D to the reactorR functioning as a flow channel by at the same time retaining theirtemperature such that the base materials and dopants B, D remain in avaporous form. The ratio between the base materials and dopants can beadjusted by changing the ratio of the gas flows B, D, for example, bymeans of adjustable valves, such as mass flow controllers, or some otherappropriate manner.

In reactor R the gas flow B of base materials and the gas flow D ofdopants is mixed (in FIG. 1 at point M) together by combining them asthe gas flow BD of reactants. Alternatively, combining and mixing thegas flows B, D can have been performed already before the reactor R. Itis obvious to a man skilled in the art that the pipelines and the likeconveying the gas flows, B, D, BD, as well as the walls of reactor R areadvantageously warmed in order to prevent the reactants fromsignificantly condensing on their walls.

Instead of the conventional heated pipelines and combining nozzlesplaced in the oven-like reactor R according to FIG. 1 or correspondingsolutions obvious to a man skilled in the art, it is possible to useplasma gas created, for example, by means of electric arc, in whichplasma gas functioning as a carrier gas the gas flows of the basematerials and dopants are mixed, in heating and mixing the gas flows B,D of the base materials and dopants.

According to an embodiment of the invention, the hot gases/vapours ofthe gas flow BD, which are mixed together and in a reduced form in thereactor R, are oxidized and thus at the same time condensated very fastinto oxides forming glass material. Oxidation/condensation is performedin such a temperature, wherein a multiple supersaturated state is formedto all reactants. Thus the condensation takes place instantaneously sothat when all the reactant and dopant components are in a supersaturatedstate, droplets are formed as a result of condensation andinstantaneously further glass particles P, whose mutual and innercomposition is homogeneous. The inner homogeneous composition of theparticles P refers here to that the different components are equallydivided in relation to the entire volume of the particles withoutlayered or other type of locally inhomogeneous structures. The ratio ofthe concentrations of the reactants in the particles is determinedsubstantially according to what the ratio of the concentrations of thereactants was in the gas phase in the gas flow BD before condensation.

It is clear to a man skilled in the art that because the material is aglass-like material, which has no clear melting or solidificationtemperature, the term “condensation” should here be understood widely.In other words, depending on the situation, either a liquid or solidglass particle can be understood to form as a result of condensation.

In order to understand the embodiment of the invention described above,it is important to note that the saturated vapour pressure of theoxidized forms of the reactants, in a certain temperature beingexamined, is significantly lower than in connection with correspondingreduced forms. Because of this, the fast condensation of the reactantsin the gas phase can be performed by mixing fast oxidative gases to thegas flow of the reduced reactants.

According to the advantageous embodiment of the invention presented inFIG. 1, the condensation/oxidation is performed by conducting intensivejets O of oxidative gases to the reactor, which jets are advantageouslylocated transverse to the gas flow BD of the reactants. Preferably, jetsO of oxidative gases are further located on the two opposite walls ofthe reactor according to FIG. 2 in such a manner that the gas jets O,which are opposite and adjacent in the cross-direction of the reactor,are located overlapping each other. This intensifies the turbulencecreated by the jets O of oxidative gases to the gas flow BD of thereactants, which turbulence mixes the oxidative gases O and the reactantgases BD effectively together. Jets O of oxidative gases can also bearranged onto more walls of the reactor R, or they can be directed insome other manner promoting turbulence and mixing in relation to the gasflow B, D of reactants.

For example, oxygen or carbon dioxide can be used as oxidative gases.Oxidative gases O can, when entering the reactor, be in the sametemperature as the reactant gases in their reduced form, in other wordshot. Thus, the condensation is mainly caused by the change in the vapourtension experienced by the reactants when they oxidize to oxides.Advantageously the oxidative gases are, however, “cold”, whichintensifies and accelerates the condensation.

In the reactor R there have been arranged such conditions, wherein theoxidation of reactants BD can take place in reaction temperatures, whichare typically around 1000-2000° C. In these temperatures, the progressof chemical reactions is determined by the mixing rate of gases. Inpractice, when the gas flow BD of reactants meets the jets O ofoxidative gases in the reactor R, the oxidation takes place in themixing zones (reaction zones) forming on the junctions between these gasflows, the “thickness” of which zones is typically around a fewmillimetres. The reactor R can be used in normal pressure, but in orderto intensify the reactions, the pressure of the reactor, the flow rateor the reactants and oxidative gases, as well as the temperature of thereactor can be adjusted in order to optimise the process.

In an embodiment of the invention shown in FIG. 3 in principle,condensation is induced by means of adiabatic expansion of the gas flowBD of reactants. In other words, the gas flow BD of reactants isdirected, for example, through a so-called Lavall nozzle LR, well knownas such. In the Lavall nozzle LR functioning as a flow channel and areactor, the gas flow BD can be accelerated to supersonic velocity. Theoxidative gases O required in the oxidation of reduced reactants can bedirected to the gas flow BD of reactants, for example, in the narrowestpart of the nozzle LR, in which case the turbulence caused by theexpansion of gases will intensify the mixing of gases. It must be notedthat in FIG. 3 the form of the Lavall nozzle LR shown for the purposesof illustration does not necessarily correspond to the exact shape ofthe nozzle used in reality.

When using adiabatic expansion, a high velocity for particles P isreached as an additional advantage, which can be utilized inintensifying the collection of particles onto the substrate by utilizingimpaction mechanism.

With an appropriate selection of oxidative gases it is possible toprevent impurities from condensing and ending up in particles. Forexample, different kind of mixtures of carbon monoxide, carbon dioxideand water can be used as oxidative gases.

The structure of the reactor R, LR can be oven-like in such a mannerthat the walls of the reactor are heated. Advantageously, materialsresistant to high temperatures, such as quartz, are used as the materialof the reactor. The walls of the reactor can be partly or entirelyporous, in which case, for example, different kinds of shielding gasescan be directed through the walls inside the reactor. The shape of thecross section of the flow channel formed by the reactor R, LR can be arectangle, circle, or some other shape appropriate for the purpose.

When forming doped glass materials, it is also possible to usechlorine-free reactants, such as TEOS (tetraethylortosilicate) or GEOS(tetraethoxygermanium) in an appropriate form as base materials B. Inaddition to the ones mentioned above, it is possible to use also otherrare earth metals and lantanides as dopants D, such as, for example,neodymium, and further also phosphorus, boron and/or fluorine.

The glass particles formed by using the method according to theinvention can be collected according to prior art onto an appropriatesubstrate, for example, around a rotating mandrel or on a planarsubstrate, on which surface is thus formed a porous glass layer, whichcan in later stages of process be sintered into a compact glass layer.The glass particles can, however, also be collected by other means, forexample as dusty powder, which can later be used as desired in preparingglass components.

It is, of course, obvious for anyone skilled in the art that bycombining the modes of operation presented above in different ways inconnection with different embodiments of the invention, it is possibleto provide various embodiments of the invention in accordance with thespirit of the invention. Therefore, the above-presented examples mustnot be interpreted as restrictive to the invention, but the embodimentsof the invention can be freely varied within the scope of the invention.

In the drawings, only the parts and components important forunderstanding the principle of the invention are presented, and it isobvious that, for example, in order to adjust the temperature andpressure conditions of the reactor R, LR, as well as gas flows, certaincomponents obvious to a man skilled in the art are required, but notshown in the figures.

1. A method for the preparation of doped oxide material from a firstreactant and a second reactant, said first reactant comprising silicon,and the second reactant comprising a rare earth metal, the methodcomprising: bringing said first reactant and said second reactant into agas phase by heating said first reactant and said second reactant;mixing said first reactant and said second reactant together to create agas flow; and mixing said gas flow with at least one oxidant gas to formparticles by oxidizing silicon and said rare earth metal, and bycondensing oxide vapors formed by said oxidizing, so that said oxidevapors reach a supersaturated state substantially simultaneously,wherein said particles are formed such that there is no time to reach achemical phase equilibrium, and all substances present in said gas floware substantially in the gas phase prior to said oxidizing, wherein oneor more jets of said at least one oxidant gas are directed to said gasflow, and wherein said one or more jets of said at least one oxidant gasare transverse with respect to said gas flow.
 2. The method according toclaim 1, wherein said second reactant is in a liquid solution prior tosaid heating.
 3. The method according to claim 1, wherein said rareearth metal is selected from a group consisting of erbium and neodymium.4. The method according to claim 1, wherein said first reactantcomprises a compound selected from a group consisting of silicontetrachloride and tetraethylortosilicate.
 5. The method according toclaim 4, wherein said particles comprise an element selected from agroup consisting of aluminium, phosphorus, boron, and fluorine.
 6. Amethod for the preparation of doped oxide material from a first reactantand a second reactant, said first reactant comprising germanium, and thesecond reactant comprising a rare earth metal, the method comprising:bringing said reactants into a gas phase by heating said reactants;mixing said reactants together to create a gas flow; and mixing said gasflow with at least one oxidant gas to form particles by oxidizinggermanium and said rare earth metal, and by condensing oxide vaporsformed by said oxidizing, so that said oxide vapors reach asupersaturated state substantially simultaneously, wherein saidparticles are formed such that there is no time to reach a chemicalphase equilibrium, and all substances present in said gas flow aresubstantially in the gas phase prior to said oxidizing, wherein one ormore jets of said at least one oxidant gas are directed to said gasflow, and wherein said one or more jets of said at least one oxidant gasare transverse with respect to said gas flow.
 7. The method according toclaim 6, wherein said first reactant comprises a compound selected froma group consisting of germanium tetrachloride and tetraethoxygermanium.8. A method for the preparation of doped oxide material from a firstreactant and a second reactant, said first reactant comprising silicon,and the second reactant comprising a rare earth metal, the methodcomprising: bringing said first reactant and said second reactant into agas phase by heating said first reactant and said second reactant;mixing said first reactant and said second reactant together to create agas flow, wherein the gas flow is directed through a de Laval nozzle;and mixing said gas flow with at least one oxidant gas to form particlesby oxidizing silicon and said rare earth metal, and by condensing oxidevapors formed by said oxidizing, so that said oxide vapors reach asupersaturated state substantially simultaneously, wherein saidparticles are formed such that there is no time to reach a chemicalphase equilibrium, and wherein all substances present in said gas floware substantially in the gas phase prior to said oxidizing.
 9. Themethod according to claim 8, wherein one or more jets of the at leastone oxidant gas are directed to the gas flow in a narrowest part of thede Laval nozzle.
 10. A method for the preparation of doped oxidematerial from a first reactant and a second reactant, said firstreactant comprising germanium, and the second reactant comprising a rareearth metal, the method comprising: bringing said reactants into a gasphase by heating said reactants; mixing said reactants together tocreate a gas flow, wherein the gas flow is directed through a de Lavalnozzle; and mixing said gas flow with at least one oxidant gas to formparticles by oxidizing germanium and said rare earth metal, and bycondensing oxide vapors formed by said oxidizing, so that said oxidevapors reach a supersaturated state substantially simultaneously,wherein said particles are formed such that there is no time to reach achemical phase equilibrium, and wherein all substances present in saidgas flow are substantially in the gas phase prior to said oxidizing. 11.The method according to claim 10, wherein one or more jets of the atleast one oxidant gas are directed to the gas flow in a narrowest partof the de Laval nozzle.